Tunable optical sources are commonly used for applications in the ultraviolet, visible and infrared regions of the optical spectrum. The output frequency or wavelength of such a source may be tuned in a continuous or stepwise fashion. In one exemplary application, single-frequency, extended cavity semiconductor lasers (ECL's) with a tuning range of approximately 40 nanometers around a center wavelength of 1540 nanometers have been developed for telecommunications applications. In many applications, it is often desirable to lock the output frequency of a tunable optical source to a predetermined frequency. In a telecommunications system, for example, the output wavelength may be locked to a single frequency in a predetermined International Telecommunications Union (ITU) frequency grid.
Frequency or wavelength lockers have heretofore been provided. The use of an etalon in a wavelength locker is described in U.S. Pat. No. 6,005,995 of Chen et al. Properties of Fabry-Perot etalons are discussed in “Angle-tuned etalon filters for optical channel selection in high density wavelength division multiplexed systems,” by A. Frenkel and C. Lin, Journal of Lightwave Technology, Vol. 7, pp. 615-624 (1989). See also §4.1 of Optical Electronics, A. Yariv, 4th Ed., Saunders College Publishing, Philadelphia, 1991.
Unfortunately, etalons and other reference elements with multipeaked transmission spectra used in frequency or wavelength lockers typically require an additional measurement system to distinguish between transmission peaks. Systems that provide this additional measurement functionality have been provided. See, for example, International Publication Number WO 01/08277 assigned to New Focus, Inc. which discloses a channel selector and an etalon. An alternative approach is disclosed in International Publication Number WO 01/11738 assigned to Coretek, Inc. wherein a first etalon is used as a wavelength locker and a filter and second etalon are used to determine a single known wavelength that can be used to calibrate the transmission profile of the first etalon. A disadvantage of this approach is that the system must tune back to the single reference wavelength in order to recalibrate if a particular instability in the source causes discontinuities in the source wavelength control. U.S. Pat. No. 5,970,076 of Hamada discloses identifying peaks in an etalon transmission using the combination of a fiber grating, optical coupler and an etalon with a large free spectral range. Disadvantages of this approach include a comparatively limited range of operating frequencies and a large degree of optomechanical complexity.
Frequency or wavelength lockers using reference elements with monotonic transmission functions have been disclosed. See, for example, U.S. Pat. No. 6,043,883 of Leckel et al., which discloses a wavemeter device that uses the dispersive phase retardation of a polarized input beam to generate a monotonic transmission function. A limitation of this device is that the derived error signal has a shallow slope that is not optimal for frequency locking to a grid of narrowly spaced frequencies because environmental and electronic noise act to limit the ability to distinguish between and lock to individual grid frequencies.
U.S. Pat. No. 6,134,253 of Munks et al. discloses an apparatus wherein the input beam from the device to be frequency or wavelength locked is split into two secondary beams that each incident on a filter. The apparatus disadvantageously relies on actively tuning one or both of the filters. A wavelength locker is disclosed in U.S. Pat. No. 6,122,301 of Tei et al. that relies on the measurement of the light power reflected and transmitted by a dielectric reflector with a transmission function that varies monotonically with wavelength. Unfortunately, the error signal that can be derived from this optical apparatus has a shallow slope that is not optimal for wavelength locking.
In general, it is an object of the present invention to provide an apparatus for frequency tuning and locking for use with an optical tunable source.
Another object of the invention is to provide an apparatus for frequency tuning and locking that separately optimizes the locking and tuning signals.
Another object of the invention is to provide an apparatus for frequency tuning and locking of the above character which has a relatively high immunity to external environmental forces such as mechanical shock and vibration.
Another object of the invention is to provide an apparatus for frequency tuning and locking of the above character which is suitable for use in an optical telecommunications system.
Another object of the invention is to provide an apparatus for frequency tuning and locking of the above character that is of minimal size.
Another object of the invention is to provide an apparatus for frequency tuning and locking of the above character that has a minimal component count.
Another object of the invention is to provide an apparatus for frequency tuning and locking of the above character that may be manufactured at a reduced cost.